Of the various light sources suitable for photoimaging including red, green and blue, blue light is the most economical and efficient. In general, the process of preparing a master printing plate having a photoresist layer involves passing energy from a light source through non-imaged portions of a modulator, e.g. a film imaged in a color which absorbs the transmitted light and transmits light radiation passing through non-imaged areas to the printing plate coated with a material which is imageable in discrete exposed areas upon exposure to radiant energy. Because of the ready availability and economy of blue light energy sources and the numerous films which are sensitive to blue light, extensive research has been directed to finding compounds which are imageable to a permanent yellow or orange yellow hue since such imaged compounds most efficiently absorb blue light and provide the highest duplicating properties. A yellow imaged modulating film would be capable of transferring energy from its non-imaged areas in a negative configuration of its pattern to a printing plate having a photoresist layer whereupon the photoresist layer develops color or is polymerized in the negative pattern dictated by the modulating film in the highest degree of acuity. Accordingly, a negative or positive of the desired image can be developed on the printing plate depending upon the yellow pattern or design inscribed on the modulating or master film.
Polyacetylenic compounds have enjoyed great popularity in imaging processes. However, it is well recognized that thermochromic behavior of polyacetylenic compounds is unpredictable and has been found only in certain narrow classes of this species. Further, within these classes, formation of a permanent yellow hue is extremely rare. While a few polyacetylenic monomers are capable of providing a yellow image at excessively high temperatures, this thermochromic effect is ephemeral, so that upon cooling, the image reverts to an initial transition color of darker shades e.g. red, blue, bronze, etc. (U.S. Pat. No. 3,501,303). Such color reversible compounds are useful as temperature or time-temperature indicators dependent on thermal changes but are not practical for use as modulating films in the preparation of printing plates, circuit boards and the like.
Accordingly, it is an object of this invention to provide a permanent yellow imaged film for use as a reverse image transmitting agent or master film in the production of printing plates and circuit boards.
Another object is to provide a commercially acceptable and economical chemically processless method for preparation of a permanent yellow imaged film.
Another object is to provide a commercially acceptable and economical process for using a permanent yellow imaged recording film as a master film in the production of printed circuit boards or master printing plates.
These and other objects of the invention will become apparent from the following description and disclosure.